The present invention relates to liquid chillers. More particularly, the present invention relates to relatively large tonnage centrifugal chillers in which so-called hybrid bearings are employed and in which the lubrication of such bearings is by the refrigerant which comprises the chiller's working fluid. With still more particularity, the present invention relates to oil-free, direct drive centrifugal water chillers capable of achieving optimized part load performance and in which the cooling of the chiller's compressor drive motor is enhanced.
Refrigeration chillers are machines that use a refrigerant fluid to temperature condition a liquid, such as water, most often for purposes of using such liquid as a cooling medium in an industrial process or to comfort condition the air in a building. Refrigeration chillers of larger capacity (from two hundred or so to thousands of tons of refrigeration) are typically driven by large centrifugal compressors. At lower capacities, compressors of the screw, scroll or reciprocating type are most often used in water chiller applications.
Centrifugal compressors are compressors which, by the rotation of one or more impellers in a volute housing, compress a refrigerant gas for use in the chiller's refrigeration circuit. The impeller or impellers of a centrifugal compressor, the shaft on which they are mounted and, in the case of so-called direct drive compressors, the rotor of the compressor drive motor, weigh hundreds if not thousands of pounds. The high speed rotation of such physically large and heavy chiller components at several thousand RPM results in unique and challenging bearing lubrication issues, particularly at start-up when these components are at rest, but also during chiller shutdown when these components coast to a stop.
Centrifugal compressors are of the direct drive or gear drive type. Hence, the chillers in which such compressors are used are generally referred to as direct drive chillers or gear drive chillers.
In direct drive chillers, the rotor of the compressor's drive motor is mounted directly to the shaft on which the compressor's one or more impellers are mounted. That shaft, in turn, is typically mounted for rotation in one or more bearings which are in need of lubrication when the chiller is in operation.
In gear drive centrifugal chillers the shaft on which the one or more impellers are mounted is driven through a series of gears rather than by the direct mounting of the rotor of the compressor drive motor to the shaft on which the impellers are mounted. The gears of a gear drive chiller act to increase the speed of rotation of the impeller beyond that of the motor which drives the impeller and in so doing increase the refrigeration effect or capacity of the chiller. In the case of a gear drive chiller, both the drive gears and the bearings in which the impeller shaft rotates require lubrication, heretofore by oil, and both direct drive and gear drive chillers have most typically employed induction motors, the speeds of which are typically limited to 3600 RPM.
It can generally be stated that chillers of the direct drive type are quieter and more efficient than chillers of the gear drive type. Further, chillers of the direct drive type are viewed as being more reliable than present day chillers of the gear drive type for the reason that chillers of the gear drive type make use of multiple gears, more bearings and other rotating parts, not found in a direct drive chiller, which are susceptible to breakage and/or wear. Gear drive chillers do, however, offer certain advantages in some applications, including, in some instances, a cost advantage over direct drive chillers.
In the cases of both direct drive and gear drive large tonnage centrifugal chillers, lubrication of their rotating components has historically proven both challenging and expensive and has been exclusively or at least fundamentally accomplished by the use of oil as the lubricant. The need for such lubrication systems has vastly complicated the design, manufacture, operation, maintenance and control of centrifugal chillers of both the direct drive and gear drive type and has added great initial and operational cost to them.
Elimination of oil as a lubricant in a large tonnage centrifugal refrigeration chiller system and the use of the refrigerant which comprises the chiller's working fluid for that purpose offers potentially tremendous advantages. Among those advantages are: elimination of many chiller failure modes associated with oil-based chiller lubrication systems; elimination of so-called oil migration problems associated with the mixing of oil and refrigerant in such chiller systems; enhancement of overall system efficiency by eliminating the oil-coating of heat exchange surfaces that results from the entrainment of oil in system refrigerant and the carrying of that entrained oil into a chiller's heat exchangers; elimination of what is viewed as an environmentally unfriendly material (oil) from the chiller system as well as the problems and costs associated with the handling and disposal thereof; and, elimination of a great number of expensive and relatively complex components associated with chiller lubrication systems as well as the control and maintenance costs associated therewith.
Further, the elimination of oil as a lubricant in a centrifugal chiller system suggests the possibility of a centrifugal chiller that offers the advantages of direct drive machines yet which, by virtue of variable speed operation, is fully the equal of or superior to gear drive machines. Heretofore, particularly good part load efficiencies have been achieved in gear drive machines by the use of specially configured gear sets capable of driving a chiller's impeller at relatively very high and/or optimal speeds. As was noted earlier, however, gear drive machines do not offer many of the advantages of direct drive machines and their use brings several distinct disadvantages, the need for an oil-based lubrication system for the purpose of ensuring the adequate lubrication of the gear train being one of them.
There have been and continue to be efforts to eliminate the need for oil-based lubrication systems in centrifugal chiller applications. Such efforts have, however, heretofore focused primarily on specialized small capacity refrigeration machines in which the bearing-mounted shaft and impeller are relatively very small and lightweight and on the use of hydrostatic, hydrodynamic and magnetic bearings in applications where bearing loads are relatively very light. In that regard, hydrostatic and hydrodynamic bearings are journal-type bearings which, while relatively low cost, simple and technically well understood, are intolerant of the momentary loss or reduction of lubricant flow. The intolerance of such bearings to the loss or reduction of lubricant available to them is exacerbated in a refrigerant environment. Further, such bearings detract from the efficiency of the compressor's in which they are used as a result of the frictional losses that are inherent in such bearings as compared to the frictional loses associated with rolling element bearings.
While hydrodynamic and hydrostatic bearings lubricated by refrigerant may have been at least prospectively employed in specialized, relatively physically small capacity compressors, the use of such bearings in large tonnage centrifugal chillers poses significant difficulties due, among other things, to the masses and weights of the chiller impellers and shafts that must be rotationally started and supported in that application. The sizes and weights of such components are such as to present significant design difficulties, particularly at chiller start-up and shutdown and during momentary loss of lubricant flow, which are yet to be overcome in the industry.
Further, even if such design difficulties are capable of being overcome with respect to the use of refrigerant-lubricated hydrostatic or hydrodynamic bearings in large tonnage refrigeration chillers, the efficiency penalties incurred in the use of such bearings due to the inherent frictional losses associated with them is disadvantageous. That disadvantage becomes larger and larger as real world issues, such as global warming, drive the need for energy consuming equipment to operate more efficiently.
Still further, the employment of hydrostatic bearings is additionally disadvantageous as a result of the need in such systems for a pump by which to deliver relatively very high pressure liquid refrigerant to such bearings in the absence of oil, the bearings of such pumps themselves requiring lubrication in operation. Such high pressure pumps are seen to be subject to breakdown arid, potentially, pose an issue of chiller reliability where hydrostatic bearing arrangements are attempted to be used.
Even further and more generally speaking, the employment of liquid refrigerant to lubricate bearings of any type in the absence of oil in a chiller system presumes the reliable availability of a supply of refrigerant in the liquid state whenever the compressor is operating and the ability to deliver such refrigerant to the bearings. However, there is essentially no single location within a chiller that contains liquid refrigerant that is capable of being delivered to such bearings under all prospective chiller operating conditions in a form or state that is appropriate for bearing lubrication. In that regard, when a chiller is shutdown and even at very low load conditions, liquid refrigerant will tend to be most reliably available from the evaporator. When the chiller is operating at load, the condenser is the most reliable source of liquid refrigerant. Therefore, the prospective lubrication of bearings by liquid refrigerant requires that an assured source of liquid refrigerant be provided for whether the chiller is shutdown, starting up, under very low load, operating at load or is coasting to a stop after it is shutdown.
An exciting opportunity exists, (1.) to achieve all of the advantages offered by direct drive centrifugal chillers, (2.) to simultaneously achieve enhanced part load chiller efficiencies, (3.) to eliminate the use of oil-based lubrication systems and (4.) to increase overall chiller efficiency, in the prospective use in refrigeration chillers of rolling element, as opposed to journal-type bearings, where the rolling element bearings are lubricated only by the refrigerant which comprises the chiller's working fluid. The possibility of eliminating oil as a lubricant in centrifugal chiller systems has become a reality with the recent advent of so-called hybrid rolling element bearings in which at least the rolling elements thereof (which are significantly less expensive than the bearing races to fabricate), are fabricated from a ceramic material. Although such bearings have been commercially available for a few years and although there has been speculation with respect to the possibility of their use in relatively very small refrigeration chillers, their actual use has primarily been in machine tool applications and in such applications, lubrication of such bearings has been and is recommended by the bearing manufacturer to be by the use of grease or, preferably, oil.
Certain of the characteristics of such bearings have, however, suggested to applicants the possibility of a large capacity centrifugal refrigeration chiller which eliminates the use of oil as a lubricant and the substitution of the chiller's working fluid therefor, even with respect to bearing lubrication. Further, such bearings are particularly well suited for high and variable speed operation as a result of the relatively lower mass of ceramic rolling elements as compared to their steel counterparts, such reduced mass resulting in reduced centrifugal forces within hybrid bearings at high speeds which, in turn, results in a reduction in the forces the bearing races must withstand during high speed operation. The use of the chiller's working fluid as the lubricant for such bearings and the need to ensure the availability of such liquid for that purpose from one source or another under all chiller operating conditions does, however, present many new and unique challenges that must be overcome.